AbstractThree lakes and catchment runoff in the Lake Gårdsjön watershed on the Swedish west coast have been sampled for methylmercury during one year. The analyses indicated that high outputs of methylmercury were present during autumn in flushing water systems with short retention time, whereas smaller outputs and variations occurred in lake waters with longer retention time.Thus, methylmercury concentration were measured as 0.16 to 0.41 ng/L (humic rich lake), 0.12 to 0.26 ng/L (limed lake),<0.1 ng/L (acid clearwater lake with long retention time) and 0.13 to 0.28 ng/L (acid runoff from a forest).A flux estimate from one forested catchment (3.6 ha) gave a yearly output of 5.2 mg and the specific output was estimated as 144 ± 70 mg/km2/y.The measured methylmercury concentration in this study accounted for only 6 to 13 of the total dissolved organic bound mercury in the water sampl
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